Many medicament delivery devices on the market are working with different types of drive means that are capable of, upon activation, acting on a stopper inside a medicament container for expelling a dose of medicament through a dose delivery member. The most common type of drive means that is used is a spirally wound compression spring.
A compressed compression spring may provide a force that is large enough to overcome the so called break loose force of the stopper, i.e. release the stopper from the inner wall of the medicament container and to be able to press the medicament in liquid form through a rather small passage in the delivery member, possibly within a predetermined time. Further, the spring at its full extension, which usually is not the full decompression, may provide a force that is large enough to complete the dose delivery, but it may also cause a breakage of the glass medicament container.
Some types of substances that are to be delivered have high viscosity, and because very small passages of the delivery member often are used, a rather large force is required. On the other hand the devices that utilize compression springs are often delivered to a user with the springs in compressed and a ready-to-use state because it is often difficult to tension the spring manually by a user and/or the device becomes very bulky if it would be provided with a spring tensioning mechanism.
Due to the pre-tension of the compression spring and in particular the very strong springs that are needed for high viscosity substances, there is a pronounced risk of breakage of the device when stored due to changes in the material, usually plastic, such as creeping and ageing. In that respect compression springs are not ideal.
In order to solve this, some devices utilize torsion springs that are tensioned by a user upon use of the device, i.e. the springs need not to be pre-tensioned, at least not to any larger degree. One device utilizing a torsion type of spring is disclosed in the document WO 2010/029043 where such a spring is wound around and connected to a drive member, capable of, upon activation, driving a drive nut such that a plunger rod, operably connected to the drive nut, is moved in a proximal direction whereby a dose of medicament is delivered through a medicament delivery member. The activation of the device and thus the release of the spring is performed by an actuation member that is positioned on a side surface of the housing of the device and slid in the proximal direction. However for some applications the position of the actuation member on a side surface is not ideal and for many users and also depending on where a preferred dose delivery site is positioned on the users body, it is preferred that the actuation member is positioned in a distal end area of the device.
UK 2437924 also relates to a torsion spring device having the activation button arranged on a side surface of the housing.
There is thus room for improvements of this type of medicament delivery device.